As head disk spacings become progressively smaller to enhance linear and areal recording densities it has been found that the use of a liquid bearing to replace the air bearing that separates the surfaces enables more reliable near contact operation. A problem that remains to be solved in near contact magnetic recording is how to maintain a uniform film of liquid lubricant between the head and disk at all times. If the separation of head and disk can be maintained with precision, high system reliability can be achieved.
If a hydrodynamic bearing is used, the sliding height will be a function of temperature since the lubricant viscosity is thermally dependent. This situation is not conducive to obtaining maximum recording densities wherein the read/write electronics are optimized for a constant head to disk spacing. A flooded hydrodynamic bearing also requires that a layer of lubricant much thicker than the actual head to disk spacing be applied to keep new lubricant flowing radially into the track being used. This results in more drag on the slider as excess lubricant must be pushed away from the load bearing feet (accelerated up to 10 to 20 meters per second). The pressures required for a hydrodynamic bearing to operate flooded in a liquid lubricant with a sliding height of only 50 angstroms are relatively high (greater than 1000 psi).
In a system that recirculates the liquid lubricant, the lubricant will dissolve plasticizers and other compounds from the file components and carry them to the disk. Since the disk is warmer than the cover of the file, evaporation occurs from the rapidly spinning thin film of lubricant covering the disk. The lubricant on the disk may become saturated with such dissolved compounds which may precipitate out and permit residues to build up on the disk if precautions are not taken.
A device using a slider that has a bearing area large enough to ski on top of the lubricant film will still have a sliding height that is viscosity dependent since the bearing surface penetrates the film slightly. In the current state of the art, the lubricant is supplied from the inner diameter of a disk from a one pass reservoir (not recirculated). Due to limited size, a suitable lubricant must be very viscous to minimize the spin-off rate. Presently no provision is made for controlling the thickness of the lubricant film which commonly varies from about 10 to 300 angstroms. This conditions may be adequate for current magnetic films and read/write channels, but ultimately uniform head spacing will be required. Further, since the head displaces the lubricant from the track being used at a rate much greater than the resupply rate, a special user algorithm will likely be needed to keep the head from operating on any given track too long. Also it may be necessary to prohibit use of a reiterative seek pattern that might deplete lubricant on a band or given pattern of tracks.
Stiction is not a problem for low viscosity liquid bearing systems because the bearing pads are sufficiently small and the lubricant film is thick enough such that a meniscus does not form between the slider and the disk. It is the menisci having angstrom scale radii of curvature that creates the large, negative pressures which draw the normal air bearing slider into the disk with sufficient force to cause stiction.